Toxoplasmosis is a protozoan infection prevalent all over the world. It gives rise to illness mainly in babies, children or adults infected in utero. Transmission can be studied to an advantage in areas where the transmission rates are high such as in Panama where over 50% of the population becomes infected during a lifetime. We propose to study how children, adults and pregnant women become infected, how often illness is produced and how infection can be avoided. There will be two study sites, Panama City and an adjacent rural area, Altos del Jobo. By focusing on the entire transmission cycle, we propose to determine how cats become infected, and where they deposit their feces containing the Toxoplasma oocysts, how long the infectious stages survive in the soil, at what rate rodents and birds become infected, which transmit the agent to kittens, and how people contract infection not only from oocyst contaminated soil, but also from raw or undercooked meat. The cost of human illness will be recorded and will be compared with the cost-effectiveness of its prevention, whether based on interference with essential parts of the life cycle, vaccination of cats or humans, providing hygienic information, or a combination of measures. The techniques of studying transmission have been developed in retrospective studies in Costa Rica and Panama; hypotheses developed in these are to be tested in this prospective study believed, to be the first epidemiologic study of Toxoplasma since transmission has been understood. By means of making appropriate modifications, results of the proposed studies can be applied at least as working hypotheses in other countries.